Current automobile engines use an oxidation catalyst, a three way catalyst, an NOx occlusion reduction catalyst, a diesel particulate filter (DPF), etc. for purification of exhaust gases. These exhaust gas purification devices are known to be adversely affected by metal components, phosphorus components and sulfur components contained in the engine oil. Thus, it is known to be necessary to reduce these components in order to prevent the deterioration of these devices.
A zinc dithiophosphate (Zn-DTP) has been conventionally used over the years as a wear resisting and antioxidation agent for a lubricant oil for use in an internal combustion engine such as a gasoline engine, a diesel engine or a gas engine and is now still accepted as an important essential additive for such a lubricant oil for internal combustion engines.
The zinc dithiophosphate, which generates sulfuric acid and phosphoric acid upon being decomposed, however, may consume basic compounds contained in the engine oil and accelerate the deterioration of the lubricant oil with the result that oil change intervals are extremely short. Additionally, the zinc dithiophosphate tends to form a sludge when subjected to high temperature conditions and to cause deterioration of the property to clean the inside of an engine. Moreover, the zinc dithiophosphate which contains, in the molecule thereof, a large amount of phosphorus and sulfur components in addition to a metal (zinc) component is considered to cause an adverse influence on an exhaust gas purifying device. In this circumstance, it is desired to develop a lubricant oil composition which excels in a wear resisting property without use of the zinc dithiophosphate.
With a view toward solving these problems, various lubricant oil additives and lubricant oil compositions have been hitherto proposed. For example, Patent Documents 1 to 3 disclose lubricant oil additives and lubricant oil compositions which contain as a principle component a disulfide compound having a specific structure. Patent Document 4 discloses a triazine compound as a lubricant additive. Further, Patent Document 5 discloses a lubricant oil which contains a thiadiazol compound.